Embercleave
Legendary Artifact — Equipment
Flash
This spell costs 
less to cast for each attacking creature you control.
When Embercleave enters, attach it to target creature you control.
Equipped creature gets +1/+1 and has double strike and trample.
Equip 
Illustrated by
Throne of Eldraine#120MythicEnglishLegality
Rulings
Embercleave's second ability can't reduce its cost below .
Once you announce that you're casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can't try to increase Embercleave's cost by removing attacking creatures you control.
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.